Reuse of sewage sludge as organic agricultural products: An efficient technology-based initiative

Reuse of sewage sludge as organic agricultural products: An efficient technology-based initiative
DR. SRIMOYEE BANERJEE1, Acharya Balkrishna1, Vedpriya Arya 1, Sourav Ghosh1, and Sumit Kumar Singh1
1Affiliation not available
April 3, 2022
1 Patanjali Herbal Research Division, Patanjali Research Institute, Haridwar, Uttarakhand 249404 2 Department of Applied and Allied Sciences, University of Patanjali, Haridwar, Uttarakhand.
*Corresponding Author
Abstract
Rapid urbanization has led to a dramatic increase in sewage generation and sludge production in turn. There are very limited methods of disposal and usage of sludge which are associated with drawbacks of energy intensiveness and economic expenses. In the current study, sludge from the Jagjeetpur sewage treatment plant (STP) has been collected, composted, and characterized. A comparison of STP sludge compost and compost made from farmyard manure showed the pH to be 6.67 which is within the prescribed limit. The phosphate content was 1.013% which is almost double the desired concentration. Few other parameters were weak in comparison but drastic changes were seen upon analyzing the fortified products. For the first product, Jaivik Poshak, the metal concentrations were highly reduced after a dilution effect due to fortification. The concentration of nine nutrients was higher in modified Jaivik Poshak. For Jaivik Khad, the second product, almost all physicochemical parameters were matched with the routine product and the Nitrogen and Phosphorous content was higher than the prescribed limits making it more suitable for fertilizer use. Phosphate was highly enriched in Jaivik Prom due to the fortification. The Zinc, Boron, Manganese, and Sodium were found to be higher in the amended product. Thus, it can be said that sludge can be converted into valuable agricultural products with minimal energy expenses and monetary benefits in addition. This can be a viable option for the management of bulk quantities of sludge in an eco-friendly manner to generate valuable returns for farmers and sustain the environment too.
Keywords: Sludge management, Organic farming, Agriculture
Introduction
The semi-solid slurry generated from various industrial processes, from wastewater treatment and on-site sanitation systems is known as sludge. It may be generated as a settled suspension from drinking water treatment, as sewage sludge from wastewater treatment, or as fecal sludge from septic tanks. After treatment of wastewater in the treatment plant, 99% of the water is recovered which is discharged as rejuvenated water. The remaining 1% which comprises solids is the sludge.
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Treatment of wastewater includes several stages. During preliminary treatment which involves the removal of large particles like sticks, stones, plastic, etc, the screened material is typically landfilled and does not become a part of the sludge. Primary treatment comprises gravity sedimentation and floatation during which half of the solids that enter this stage are removed and become the primary sludge. It comprises both organic and inorganic components. Secondary treatment is a carefully controlled and biologically governed process where the microbes degrade the organic components in the wastewater. The microbial biomass generated as a result of this is organic in nature and becomes the next component in the sludge. Tertiary treatment is done to reduce the nitrogen and phosphorus component, suspended solids, and biological oxygen demand. Precipitated components during the tertiary stage of treatment add to the total sludge quantity. The final step includes disinfection to kill the pathogenic microbes and discharge of the rejuvenated water (Penn state extention, 2010). On average, dry sludge contains 50-70% organic matter, 30-50% mineral content, 3.4-4% Nitrogen, 0.5-2.5% Phosphorous, and several other nutrients depending on the types of wastes treated and the stabilization processes involved (Kacprzak et al., 2017).
Owing to the rapid urbanization and industrialization, the quantity of sludge produced has become huge, and hence special attention is required for its management. One aspect of sludge management is its use in agriculture. Application of large and uncontrolled amounts of untreated sludge results in the seepage of heavy metals, phenolic compounds, polycyclic aromatic compounds into the soil, groundwater, and surrounding water bodies which may bring about drastic changes in soil fertility and damage to flora and fauna (Balkrishna et al., 2022.; Houillon & Jolliet, 2005). It is important to pre-treat the sludge, make it suitable to meet legal regulations, and then use it for a particular purpose. However, pretreatment technologies are complicated, expensive, and pose the question of the disposal of concentrated pollutants separated from the sludge. Thermal incineration is another alternative that significantly reduces the volume of sludge. The ash thereby generated as a result of incineration needs other methods for cyclization. The products of incineration may be used in cementing industry or for vitrification (Cieslik, 2015). Other methods exist for the recovery of valuable substances from the sludge. Pyrolysis of sewage sludge under anaerobic conditions can be done to generate adsorbents after the addition of various reactants. Other by-products include pyrolysis oil which can be used as fuel. In this way, partial recovery of sludge is possible (Cieslik, 2015; Samolada & Zabaniotou, 2014). Phosphorus recovery is another technology implemented by most treatment plants.
The presence of heavy metals poses a problem for almost all sludge management systems (Mailler, 2015). Recovery of such metals is done in furnaces in oxidative or reducing conditions. Metals recovered in this process are used in industries as catalysts or additions in special products. This process is mainly done in developed countries owing to the high-end instrumentation requirements, cost, and energy inputs involved (Cieslik, 2015).
Underdeveloped nations primarily dispose of the sludge at the source which thereby creates environmental problems and risks for humans and aquatic life (Balkrishna et al., 2022). The wastewater treatment processes concentrate heavy metals, organic pollutants, and pathogens. Sludge disposal can therefore bring about the release of toxic components into the environment which can thereby enter the food chain (Chen et al., 2012; Wei & Liu, 2005; Pathak, 2009). Though landfilling is an economical and low energy consuming process, it becomes a significant source of CH4 and N2O which are greenhouse gases (Peters & Rowley, 2009). Landfilling is also incapable of utilizing any of the nutrients that are present in the sludge. Landfill leachates containing P and heavy metals can also significantly affect groundwater and surface waters. Land application of sludge also has many benefits as it improves soil qualities and is inexpensive. However, land application is also limited owing to its heavy metal, micropollutant, and pathogenic microbe content (Wang et al., 2008).
The current manuscript deals with how sludge may be converted into a valuable agriculture input in an eco- friendly way. Three organic fertilizers routinely made with farmyard manure compost have been modified by using sewage treatment plant (STP) sludge compost. In our study, expensive and energy-intensive procedures have been completely avoided yet the product developed meets regulatory parameters. The optimum fortification of sludge with suitable additives led to the production of organic fertilizer with reduced
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heavy metal concentration yet increased capacity for plant growth promotion. This procedure could solve the problems of managing drastic amounts of sludge while creating valuable agricultural inputs. It is proposed as a sustainable option for converting waste into a valuable product for use in agriculture.
Materials and methods:
2.1 Site of sludge collection :
The sludge sample was collected from a sewage treatment plant at Jagjeetpur in Haridwar, Uttarakhand, India. The location coordinates are 29.9015435, 78.1376691. This treatment plant has been commissioned since November 2019 under the Namami Gange Program as a part of its interventions to prevent the flow of untreated wastewater into the river Ganga. The plant came under working conditions in June 2020. The total treatment capacity of the plant is 68 million liters per day (MLD) and uses sequential batch reactors for the treatment process. An average sludge production reported for April 2021 was 399 MT (National Mission for Clean Ganga, 2021). During our inspection to the site on 08.02.2022 and 15.02.2022, we found no tertiary treatment to be taking place in this plant. As told by the head engineer of the site, the average sludge production per day was approximately 19.68 MT. Also, the sludge generated from the plant was deposited within the plant premises near the Ganga River bank. It was found that the untreated sludge was supplied to farmers on their demand for usage as agriculture input. The head engineer of the site handed over the data regarding the inlet and outlet wastewater parameters for January and till mid-February which were collected by the auto analyzer as well as manually rechecked. With regard to sewage, the data of solid particles (in %) in the outlets of seven centrifuges for the period of January, 2022 was given. The extent of dewatering of the sewage can be understood from that. The data for MLSS in the six basins of the sequential batch reactor was also given for January, 2022 which indicated the extent of suspended solids in the sequential process and the extent of purification taking place after the waste water crossed each basin.
Sample collection and characterization :
The sludge sample was collected in bulk in sealed packets and brought to the laboratory. For the character- ization of the sample, moisture content, total organic carbon, pH, C/N ratio, Total Nitrogen content, Total Phosphate content, Total Potassium content (as K2O) and Total NPK were measured. The metal content was analyzed using Atomic Absorption Spectroscopy (AAS). All the procedures for the measurement of different parameters as well as metals were followed from Fertilizer Control Order (FCO), 1985 (Government of India, 1985). A comparative assessment was done between the metal and macro and micronutrient pa- rameters of routinely prepared compost and compost from the STP sludge to understand the nutrient status as well as heavy metals status in both the compost.
2.3 Product preparation
There are few tested products by Patanjali Organic Research Institute (PORI) which are used as organic fertilizer inputs in agriculture. They are prepared by mixing the individual components in a fixed ratio which has been deciphered by experimentation and trials. The products contain compost as one of the components which is prepared at the institute from farmyard manure. The products are developed with the purpose of increasing agricultural productivity and are under huge demand by farmers in Uttarakhand and India. In order to take a step forward in the management of the huge quantities of sludge produced at wastewater treatment plants while conforming to environmental parameters and increasing agricultural productivity, PORI has attempted to make organic fertilizer from STP sludge. Three different products have been prepared by using STP sludge from Jagjeetpur instead of in-house compost.
Patanjali Jaivik Poshak is a mycorrhiza-based patented (Patent number 201811028449) granular biofertilizer that contains a small but powerful nutrient nano compound mixture. A modified version of the existing product was prepared by mixing the ingredients by replacing the routinely used in-house compost with compost prepared from STP sludge. The mixture was made for a total of 10 Kg.
The next product is Patanjali Jaivik Khad which is a 100% organic manure made from the leftovers of medicinal plants, flowers, vegetables, and cattle dung composting procedures and is fortified withTrichoderma
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, Pseudomonas , and Aspergillus . Humic acid, amino acids, seaweed, and natural nutrients are all balanced in this product. It enables plants to defend themselves against illnesses and insects. This formulation strengthens crops’ immune systems and helps them resist disease. It can be used for paddy, wheat, sugarcane, potato, soybean, groundnuts, peas, onion, and a variety of vegetables, pulses, and fruits, among other crops. This was also made for a total quantity of 10Kg.
The third product is Jaivik Prom (Patent number 201811028448). It is a phosphorous-rich organic farming input that contains 10.42 to up to 12% phosphorous. Bio residues, rock phosphate, Khalil, amino acid, plant development factors, and helpful micro bacteria makes up this patented product. Paddy, maize, wheat, sugarcane, potato, soya, groundnut, peanut, onion, and numerous vegetables, legumes, fruit, flowers, and medicinal plants can all benefit from the application of this product. This particular product has also been prepared by substituting the in-house compost with STP sludge compost. The final 10kg product thereby prepared was compared with the original product.
All three products are routinely produced using in-house compost made from farmyard manure. They are all patented. The compost component was replaced by the STP sludge. After mixing in the desired proportions the parameters were analyzed for the routinely made products and the modified products. A comparative analysis was done to understand the effect brought about with the substituted compost.
In order to characterize the product quality made with the STP sludge, a comparative analysis was done to understand the effect of the fortification process. The metal and nutrient status of the standard product and the modified products was compared.
Results and discussions
3.1 Sampling site
The 68 MLD Sewage treatment plant was found to be a well-functioning and maintained plant in Jagjeetpur, Uttarakhand, India. It was effectively cleaning wastewater generated from the city and its surroundings and thereby preventing Ganga River from pollution due to numerous contaminants. Two representative images of the STP plant are shown below in figure 1.
Figure 1: Representative images of Jagjeetpur STP
The sludge generated regularly was dumped within the plant premises and hence alternative solutions for its usage need to be found.
The data of soild partciles (%) in each centrifuge outlet which is an indicatorof the extent of dewatering, for the month of January 2022 was obtained from the head engineer and has been given as given as table 1 in the supplementary material.
The geometric mean of solid particles generated from the seven centrifuge outlets was plotted as a time series which is shown in Figure 2 below.
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Figure 1: This is a caption
Figure 2: Time series plot of geometric mean of the solid particles in different centrifuge outlets
From the time series plot it is evident that there are certain ups and downs as the geometric mean of seven outlets has been taken but the overall efficiency of dewatering is within a range of 22.24 to 28.09% which is a very small range. The standard deviation is also very less with a maximum value of 4.78. It can be deciphered that the working efficiency of the plant is very high and all the centrifuges are working at comparable efficiencies. Data over a month in this range also states that the sludge generation capacity is also optimum.
Data for MLSS (mg/L) in the six basins of the SBR in January, 2022 has been given as Table 2 in supple- mentary material
The mean of each basin along with the standard deviation was plotted and is shown in Figure 3 below.
Figure 2: This is a caption
Figure 3: Average MLSS values in January, 2022 in the six basins of the SBR
From the figure, it can be seen that there is a sequential increase in the MLSS values in the six basins except for a slight fall in basin 3. Again after that, the MLSS values have continuously increased and basin 6 has
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the highest MLSS value which is the normal case for an SBR. As the wastewater purification proceeds in a stepwise manner, the water content is sequentially removed, increasing amounts of solids accumulate and hence the first basin shows minimum MLSS concentration while the last shows the maximum value. The standard deviation values also are within limits which shows that the performance of the plant over the month was highly stable and efficient.
3.2 Sample characterization
The routinely prepared in-house compost, as well as the compost prepared from STP sludge, were character- ized simultaneously to compare their characteristics. The STP sludge had certain undesirable components like grass which were could create problems in the final product. The STP sludge was ground well before analysis in order to prevent lump formation due to its sticky nature. The comparative analysis of the in- house compost and STP sludge compost along with the prescribed specifications as per FCO, 1985 is given below in Table 1.
Table 1: Comparative analysis of in-house sludge and STP sewage sludge compost
Serial number Parameter Specification Concentration in in-house compost Concentration in STP compost
1 Moisture content (%) Not more than 25% 30.551 52.109 2 Total Organic Carbon (%) Minimum 14% 15.243 15.017 3 pH 6.5-7.5 7.56 6.78 4 C:N ratio Less than 20 15.911 25.45 5 Total Nitrogen (N) Minimum 0.5% 0.958 0.59 6 Total phosphate as P2O5 (P) Minimum 0.5% 0.766 1.013 7 Total Potassium as K2O (K) Minimum 0.5% 0.37 0.164 8 Total NPK nutrient Not less than 3% 2.094 1.767
From the above comparative analysis, it is evident that there are few parameters where the STP compost is not as good as in-house compost and vice versa. The moisture content is way higher for the STP compost than the prescribed limits. The major reason for this is that the sludge was collected on a rainy day and as it is lying open in the plant premises it is quite usual for it to get wet. However, this can be easily managed with regular drying technologies. The organic carbon content is above 15% for both in-house and STP sludge compost against the specification of 14%. Interestingly, the pH of the sludge compost is within the prescribed specifications as compared to the pH of in-house compost. This is beneficial as it would not require external additions to bring the pH within the limit thereby saving cost and preserving the chemical nature of the compost. The C/N ratio for the sludge compost is much higher than the specifications which is due to the low N content. It can be seen that the N content is just marginally higher than the prescribed specification in the case of the sewage sludge but the in-house compost is of better quality in this respect. The phosphate content is almost double the specifications in the case of sludge compost. The phosphate content is approximately 0.8% for in-house compost as compared to a specification of 0.5%. The Potassium content of STP sludge compost is way lower than the recommended specification. The in-house compost also does not contain the desired concentration. The total NPK content is also lower than the specifications for both in-house and sludge compost. Overall, we find that the STP sludge contains a number of valuable nutrients. Most of them are in concentrations greater than the desired specifications. The presence of plant macronutrients also makes it an important constituent that may be used in addition to other inputs so as to utilize those for plant growth. Fortification of the sludge compost with correct additives will enhance its properties and make it a fruitful addition for agriculture.
We have tried to analyze the metal concentrations present in the sludge as well as in-house compost using Atomic Absorption Spectroscopy. The macro and micronutrient status were also measured and analyzed. The table 2 below shows the results of heavy metals in STP and in-house sludge in comparison to the desired concentration as specified by government regulations (Government of India, 1985).
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Serial Number Metal contaminant Units Specified concentration for use as agricultural fertilizer Measured concentration in in-house compost Measured concentration in STP sludge compost
1 Arsenic mg/Kg 10 3.77 3.00 2 Cadmium mg/Kg 5 0.65 0.84 3 Mercury mg/Kg 0.15 0.67 1.39 4 Lead mg/Kg 100 13.18 18.01 5 Chromium mg/Kg 50 10.61 14.15
The metal concentrations in the sludge are well below the permissible limit except for Mercury. The con- centration of Mercury is 0.67 in…